As one example of the techniques of the type described above, there is conventionally known a ball-valve type double eccentric valve disclosed in Patent Document 1 listed below. This double eccentric valve is provided with a valve seat including a valve hole and an annular seat surface formed on the edge of the valve hole, a valve element having a circular disc shape and an annular sealing surface on its outer periphery corresponding, or conformable, to the seat surface, and a rotary shaft to rotate the valve element. Herein, the axis of the rotary shaft extends in parallel to a diametrical direction of the valve element and the valve hole and also is positioned eccentrically from the center of the valve hole in a radial direction of the valve hole. The sealing surface of the valve element is positioned eccentrically from the axis of the rotary shaft toward the extending direction of the axis of the valve element. To ensure the foregoing double eccentric structure, the valve element includes a protrusion protruded from an upper surface and fixed to the rotary shaft, and the protrusion is positioned eccentrically from the center of the valve element in a radial direction thereof. Further, the outer periphery of the rotary shaft is set on the protrusion and secured thereto with a screw. By rotating of the valve element about the axis of the rotary shaft, the sealing surface is moved between a fully closed state in which the sealing is in surface contact with the seat surface of the valve seat and a fully open state in which the sealing surface is furthest away from the seat surface. In this double eccentric valve, the valve seat is provided with an elastic member, so that the seat surface of the valve seat is pressed in contact with the sealing surface of the valve element during full close to increase sealing performance in the fully closed state. When fluid pressure acts on the valve element, the valve seat is pressed against the valve element by the elastic member, thereby blocking up a gap or clearance between the valve element and the valve seat.
As other techniques, for example, there is known a butterfly-valve type double eccentric valve disclosed in Patent Document 2 listed below. In this double eccentric valve, a sealing surface of a valve element and a seat surface of a valve seat are made of metal material by buildup welding. This double eccentric valve is illustrated in schematic diagrams in FIGS. 20-22. A valve element 61 is provided, on a back side thereof, with an attachment part 64 attached to a rotary shaft 63 at a position doubly eccentric from a sealing surface 62. This attachment part 64 is fixed to the rotary shaft 63 coaxial therewith to constitute the double eccentric valve.